Posh part of London holding a mass for teacher (5)
I believe the answer is:
swami
'teacher' is the definition.
(I've seen this before)
'posh part of london holding a mass' is the wordplay.
'posh part of london' becomes 'swi' (I can't explain this - if you can you should believe this answer much more).
'holding' indicates putting letters inside.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'a'+'m'='am'
'swi' enclosing 'am' is 'SWAMI'.
'for' is the link.
Can you help me to learn more?
(Other definitions for swami that I've seen before include "Indian religious teacher" , "Teacher abroad" , "Hindu mystic" , "Religious master" , "Hindu religious instructor" .)